The difference in refractive index between a sample and a reference material is called the differential refractive index, or DRI, and is a physical parameter of considerable importance. In liquid chromatography, the DRI between a sample liquid consisting of a mobile phase plus a solute and a reference liquid, which is the pure mobile phase, is often used as a determination of solute concentration. Another physical parameter of interest is the absolute refractive index, RI, of a fluid. In general chemistry, the RI is used to identify liquids, determine the purity of liquids, and to measure high concentrations of dissolved molecules such as sugar solutions. In light scattering measurements, the RI of the mobile phase is an important input parameter in determining the molar mass and size of molecules and particles.
To date there has been no one instrument which can measure both the DRI between a sample fluid and a reference fluid, and the RI of a fluid. A typical instrument for measuring the DRI only is a “walk-off” type differential refractometer. That instrument contains a transparent cell containing two fluid chambers, and having an angled transparent window separating the chambers. A beam of light passes into the cell, through one chamber, through the angled window separating the chambers, through the second chamber, and finally out of the cell, as pictured in FIG. 1. For the cell pictured, if the fluids in the two chambers have identical indices of refraction, then after exiting the cell, the transmitted beam of light travels in a path parallel to the incident beam. If the two fluids have different indices of refraction, then the transmitted beam of light travels at an angle different from the direction of the incident beam. The angle between the incident light beam and the transmitted light beam is, to first order, proportional to the difference in refractive index between the two liquids. The angular deflection of the light beam may be measured by a variety of well-established techniques, and so the DRI of one fluid relative to the other may be measured and reported. It is noted that the fluids are usually introduced into the respective chambers by connection means providing flows transverse to the planes shown. Each chamber contains means to introduce and remove fluids contained therein, usually on opposite sides of the chambers.
A very simple, though imprecise, instrument that may be used for the measurement of the RI of a fluid is comprised of a transparent cell with only one fluid chamber. The fluid chamber has at least one wall that is oriented with respect to the incoming beam, as shown in FIG. 2. A beam of light passes into the cell, through the fluid, and exits the cell via the angled face. The transmitted beam will have an angular deflection that depends upon the index of refraction of the fluid relative to the known RI of the air or medium surrounding the cell. Embodiments that are more precise would include the well-known Abbe refractometer. As with the DRI instrument, that angular deflection may be measured by a variety of well-established techniques, and so the RI of the fluid may be reported. Note that the chambers used in the structures of FIGS. 1 and 2 are defined generally in terms of right triangles.